How to calculate effective resolution of ciruit

Discussion in 'General Electronics Chat' started by embedded.world, Jun 10, 2014.

  1. embedded.world

    Thread Starter Member

    Feb 27, 2014
    36
    0
    I am using 24 bit adc ADS1247 to measure voltage drop across thermsistor by passing current through it.
    http://www.digikey.com/product-detail/en/PR103J2/615-1069-ND/2651611

    Since thermistor value gets very large so I connected a 20K fixed resistor in parallel with it.


    1. Now my current source is 50uA with initial error is +-6% with temperature drift +-100ppm/C.

    2. Voltage reference is 2.048V with +-10mV & drift is +-15ppm/C.

    3. Resistor = 20K +-0.01% with drift +-5ppm/C.

    4. PGA = 2, operating range = -40C to +60C.

    5. Minimum voltage step need to measure = 75uV.


    Questions:
    1. How to calculate effective resolution I will get finally.

    2. Do I need any other factors to consider besides I mentioned above.

    3. Do final resolution is equal to ENOB?

    4. Will I be able to achieve minimum voltage step = 75uV with this adc




    Crosspost:
    http://www.eevblog.com/forum/beginners/how-to-calculate-effective-resolution-of-ciruit/
     
    Last edited: Jun 10, 2014
  2. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,135
    200
    Why don't you use some of the linearizion IC's, like TI's PGA309?
     
  3. dougc314

    Member

    Dec 20, 2013
    38
    11
    The thermistor has a non-linear resistance change with temperature. In order to calculate resolution you will need to generate on expression for resistance as a function of temperature. There are good articles on Wikipedia for this. Also most Thermistor vendors publish a table of resistance vs temperature for their thermistors and you could simply use the table to determine the dR/dT at various temperatures. I think the most important AD parameter you are looking for is differential non-linearity, check that out on Wikipedia as well
     
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